A lion's share of current state-of-the-art nonvolatile memory is silicon-based flash memory due to its high density and low cost. However, flash memories have several disadvantages, such as low operation speed (e.g., write/erase times of about 1 ms/0.1 ms), poor endurance (e.g., about 106 write/erase cycles), and high write voltage (e.g., greater than 10 V). Moreover, flash memories may reach the miniaturization limit in the near future due to large leakage currents.
One technology that may overcome the disadvantages of flash memories is resistive random access memory (RRAM). In general, a RRAM cell includes an insulator or semiconductor sandwiched between two conductors. The underlying physical mechanism of RRAM is usually resistive switching (RS), which allows the cell to be freely programmed into a high resistance state (HRS, or OFF state) or a low resistance state (LRS, or ON state) under external electrical stimuli. In most cases, current flows uniformly through the device in the HRS and is restricted to a local region with high conductance known as a conducting filament (CF) in the LRS. The simple structure of RRAM enables easy integration in passive crossbar arrays with a small size of 4F2 (F is the minimum feature size). The size can be further reduced to 4F2/n within vertically stacked three-dimensional (3-D) architectures (n is the stacking layer number of the crossbar array).
However, RRAMs have their own limitations. For example, current RRAMs typically use amorphous materials as the switching medium disposed between electrodes. During switching events, conductive filaments can be formed anywhere within the amorphous material. As a result, it can be difficult to accurately locate or confine the conductive filament. In addition, the random formation of conductive filaments in RRAMs can also reduce the uniformity (and increase the variance) of performance among different cells. The increased individual variability of RRAM cells can in turn limit wide spread applications.